This invention relates to the thermal printing apparatus and, more particularly, to a thermal printing electronic postage meter for printing of a postage indicia, and a method for automatically detecting a fault condition in a thermal printhead and providing remedial action in response to the detection of the fault condition depending on the type and location of the fault condition.
Metering of mail is conventionally done in either a prepayment or a post payment mode. That is, in one system of postage payment, the owner of a postage meter prepays to the post office a certain amount of funds for which he receives credit within the vault, or sometimes referred to as accounting system, of the postage meter. In another system, known as a post payment system, the mailer is charged for postage expended from the use of his meter. In either system, it is critical that proper accounting be made for the value of postage dispensed by the meter and that accounting for expended postage can be reconciled between the post office and the mailer. The post office assures itself of proper posting by referring to the numeric value which is part of the printed postage indicia. The indicia is comprised of artwork which is intended to be of such a nature as to the very difficult to counterfeit, a posting value, a posting date, and a posting location. Additionally, there is a meter identification or validation number included as part of the postage indicia. Each part of the indicia represents an added degree of security in assuring that fraudulent posting has not taken place. For example, it is very critical that the value amount is printed accurately as it represents the funds being transferred between the customer and the post office. The posting date is very important, in that, there are rules that require that the posted mail be placed in the hands of the postal service on the date of posting. The meter identification number or validation number is a means of checking that the funds transfer are being made via a authorized postage meter.
The use of digital printing techniques, particularly thermal printers, has introduced printing fault conditions which are not present in convention impacted postage meter printing techniques. For example, should the thermal printhead have a defective element or elements, depending on the location of the element or elements, substantial alteration of the information being conveyed by the postage indicia can occur. For example, a defective thermal elements in the region which is printing the posting date can make a "9" appear to an optical reading device as a "0". Or a "2" appear as a "7", or a "6" appear as a "5". Therefore, in areas where numeral values are being printed, it is very critical that the printing control circuit knows or in some way detect the presence of a defective print element in these areas, and then perform the proper corrective measures. Other areas of the indicia are less critical, such as, in the area of the artwork where a defective or an operative print element will not affect the accounting for postage printing.
As a result, it is necessary or beneficial for the control system of a digital printing system associated with an electronic postage meter to be able to detect defective print elements as a fault condition relative to the location of that defective print element or elements in the indicia to be printed and then take corrective action in response to that condition. Conventional thermal control system provides means of detecting the presence of a bad thermal element, however, in the case of barcode printing the control system simply provides a shifting of the full bar code to avoid the defective element. It is appreciated that such systems are limited in the number of shifts which can practicably be performed. Other systems which address principally alphanumeric character printing systems merely prevent further printing once a bad element is detected.
Another defect condition is associated with the thermal printhead temperature during the printing process or at startup in which the environment is very cold. In either case, the transfer of printing ink from the thermal printhead ribbon to the print medium, for example, an envelope or tape strip can be adversely affected, thereby producing a partial or distorted image. Another fault condition occurs if the printhead is operated at excessive temperatures over a period of time resulting in premature failure of the printhead.